The invention relates to an optical waveguide fiber optimized for low attenuation. In particular, waveguide fiber attenuation is minimized for any core refractive index profile by proper selection of the core refractive index profile variables.
The dependence of waveguide properties upon the configuration of the refractive index profile has been described in the pioneering patent, U.S. Pat. No. 4,715,679, Bhagavatula. In that patent, core refractive index profiles are disclosed which provide for a variety of waveguide fiber properties, especially those having a zero dispersion wavelength shifted into the 1550 nm operating window and those which have a relatively constant dispersion over an extended wavelength range such as 1250 nm to 1600 nm.
In response to demands for specialized waveguide fibers, particularly with regard to high performance waveguides, investigation of waveguide core refractive index profiles has intensified. For example in U.S. Pat. No. 5,483,612, Gallagher et al., (the ""612 patent) there is disclosed a core profile design which provides low polarization mode dispersion, low attenuation, a shifted dispersion zero, and low dispersion slope. Other core refractive index profiles have been designed to meet the requirements of applications which include the use of higher power signals or optical amplifiers.
A problem which may arise when a core profile is altered in order to arrive at a desired property is that the property is realized at the expense of another essential property. For example, a certain core refractive index profile design may provide increased effective area, thus reducing non-linear distortion of the signal. However, in this large effective area waveguide fiber, the bend resistance may be seriously compromised. Thus, core profile design is an exacting task, in which model studies usually precede the manufacturing stage of product development.
The interaction of the profile variables is such that one skilled in the art usually cannot, except perhaps in a very general way, predict the impact of a refractive index profile change upon such waveguide properties as, bend resistance, attenuation, zero dispersion wavelength, and total dispersion and total dispersion slope over a selected wavelength range. Therefore, studies of waveguide refractive index profiles usually include a computer simulation of the particular profile or family of profiles. Manufacturing testing is then carried out for those refractive index profiles which exhibited the desired properties.
In a continuation of the work disclosed in the ""612 patent, a family of profiles was found which produced a high performance fiber having a zero dispersion wavelength above a pre-selected band of wavelengths and excellent bend resistance. A description of this work has been filed recently as a provisional application, Ser. No. 60/050550.
As further model studies and manufacturing tests were completed, it became clear that:
a particular family of profiles could be found to provide a selected set of operating parameters; and, most surprisingly,
the profiles of the particular family could be further adjusted to optimize attenuation without materially changing the operating parameters.
The radii of the regions of the core are defined in terms of the index of refraction. A particular region has a first and a last refractive index point. The radius from the waveguide centerline to the location of this first refractive index point is the inner radius of the core region or segment. Likewise, the radius from the waveguide centerline to the location of the last refractive index point is the outer radius of the core segment. Other definitions of core geometry may be conveniently used.
Unless specifically noted otherwise in the text, the parameters of the index profiles discussed here are defined as follows:
radius of the central core region is measured from the axial centerline of the waveguide to the intersection with the x axis of the extrapolated central index profile;
radius of the second annular region is measured from the axial centerline of the waveguide to the center of the baseline of the second annulus; and,
the width of the second annular region is the distance between parallel lines drawn from the half refractive index points of the index profile to the waveguide radius.
The dimensions of the first annular region are determined by difference between the central region and second annular region dimensions.
Core refractive index profile is the term which describes the refractive index magnitude defined at every point along a selected radius or radius segment of an optical waveguide fiber.
A compound core refractive index profile describes a profile in which at least two distinct segments are demarcated.
The relative index percent (xcex94%) is:
xcex94%=[(n12xe2x88x92nc2)/2n12]xc3x97100, where n1 is a core index and nc is the minimum clad index. Unless otherwise stated, n1 is the maximum refractive index in the core region characterized by a % xcex94.
The term alpha profile refers to a refractive index profile which follows the equation,
n(r)=n0(1xe2x88x92xcex94[r/a]xcex1) where r is radius, xcex94 is defined above, a is the last point in the profile, r is chosen to be zero at the first point of the profile, and I is a real number. For example, a triangular profile has xcex1=1, a parabolic profile has xcex1=2. When xcex1 is greater than about 6, the profile is essentially a step. Other index profiles include a step index, a trapezoidal index and a rounded step index, in which the rounding may be due to dopant diffusion in regions of rapid refractive index change.
The profile volume is defined as 2∫r1r2 (xcex94% r dr). The inner profile volume extends from the waveguide centerline, r=0, to the crossover radius. The outer profile volume extends from the cross over radius to the last point of the core. The units of the profile volume are % xcexcm2 because refractive index is dimensionless. To avoid confusion, the profile volumes will be connoted a number followed by the word units.
The crossover radius is found from the dependence of power distribution in the signal as signal wavelength changes. Over the inner volume, signal power decreases as wavelength increases. Over the outer volume, signal power increases as wavelength increases.
The bend resistance of a waveguide fiber is expressed as induced attenuation under prescribed test conditions. A bend test referenced herein is the pin array bend test which is used to compare relative resistance of waveguide fiber to bending. To perform this test, attenuation loss is measured for a waveguide fiber with essentially no induced bending loss. The waveguide fiber is then woven about the pin array and attenuation again measured. The loss induced by bending is the difference between the two measured attenuations. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. During testing, sufficient tension is applied to make the waveguide fiber conform to a portion of the pin surface.
The bend test used in the model calculations was a single turn of waveguide fiber around a 30 mm diameter mandrel.
The effective group refractive index (ngeff) is the ratio of the velocity of light to the group velocity. The mathematical expression for ngeff in terms of electromagnetic field, refractive index, wavelength and propagation constant, derives from Maxwell""s equations, or, more particularly, from the scalar wave equation.
The propagation constant xcex2, also called the effective refractive index is an electromagnetic field parameter related to field propagation velocity and is found by solving the scalar wave equation for a selected waveguide. Because xcex2 depends upon waveguide geometry, one may expect that bending the waveguide will change xcex2. An example of a scalar wave equation descriptive of the electromagnetic fields which are supported by a particular waveguide geometry is found in xe2x80x9cOptical and Quantum Electronicsxe2x80x9d, J. P. Meunier et al., 15, (1983), pp. 77-85.
The present invention is therefore directed to an optical waveguide fiber having a core refractive index profile which produces a pre-selected set of operating properties and in which attenuation is optimized for that particular refractive index profile.
The novel core refractive index profile has a core region and a surrounding clad layer which together form a waveguide fiber. To confine light within the fiber, at least a portion of the core index profile must have a higher refractive index than at least a portion of the clad layer. Usually, the clad layer index profile is a single step, although useful designs which have a modified clad index have been made.
The core refractive index profile, defined above, is a refractive index value defined at each point along a specified portion of the waveguide radius. Thus the core index profile may be expressed as an index value n(r) at points along a radius beginning at 0, the center of the waveguide, and extending to a radius ro. This core index is designed to produce a pre-selected set of waveguide fiber operating properties. The operating properties each may have tolerance limits so that a family or set of core refractive profiles exists which produce these waveguide operating properties. Even in a model case, in which the operating properties each have a single value, a set or family of refractive index profiles which provide the properties can be found.
The set of core refractive index profiles, which provide the pre-selected waveguide operating parameters, may be specified by stating the amount of refractive index variation at any radial point r of the refractive index, xcex4n(r), and the amount of variation of the total radius, xcex4ro, which is allowable.
Through modeling studies of the family or set of allowable refractive index profiles, a subset of profiles have been found which have lower attenuation than the other members of the set. The waveguide properties which distinguish this highly preferred subset are the effective group refractive index, ngeff, and the propagation constant xcex2. In particular, the highly preferred subset of lowest attenuation refractive index profiles have the lowest ngeff of any other members of the set, and exhibit the smallest change in the square of the propagation constant, xcex22, when the waveguide is bent. Any of a number of bending models can be used to calculate the bending induced change in xcex22. A bending model used in the case described here is one in which the waveguide makes one turn around a 30 mm diameter mandrel.
The lowest attenuation refractive index profile family or set has been found for step index single mode waveguide fiber, trapezoidal shaped index, rounded step index, and compound index profiles made up of combinations of these. Thus it is believed to be very likely that essentially every family or set of profiles has members which exhibit lowest attenuation and that these members are characterized by the lowest ngeff and lowest change in xcex22 in bending as compared to any other member of the set or family of profiles.
Thus the core refractive index profile may have relative index differences, xcex94, which are positive or negative. The index profiles may have only one region of step, trapezoidal, rounded step, or xcex1-profile shape, in which xcex1 can assume any real number value. Alternatively, the core refractive index profile may be any combination or permutation of these shapes in two or more regions which are defined segments of the core region.
A particular compound core embodiment of the novel core refractive index profile is one in which N segments are defined. Each segment has a xcex94 % value and a shape. Various widths and radii (see the definitions section above) of the segments are defined until the complete geometry of the compound core has been specified. For example the outer radius, measured from the waveguide center to the outermost point of the particular core refractive index segment, of each segment may be specified. In general, the relative indexes, xcex94%, for single mode waveguide fibers are in the range 0 to 3.5% and the outer radius of the outermost segment is in the range of 1 xcexcm to 30 xcexcm. A preferred band of operating wavelengths is 1200 nm to 1750 nm, which includes the operating windows near 1300 nm and 1550 nm.
An embodiment of the invention comprises a compound core having three segments. This embodiment is discussed in detail below. The model used to calculate waveguide fiber structure and properties can be adapted to account for a refractive index dip on centerline. In the case where there is some dopant depletion from the centerline, the lower limit of xcex941% is decreased about 15%. Although dopant compensation can be made to eliminate centerline depletion, it is more time and cost efficient to adjust other profile parameters to compensate for the depletion. The definitions given above are followed in that r3 is the radius drawn to the center of the base of the third segment and that w3 is the width at the half relative index points of the third segment.
A preferred embodiment of the three segment core refractive profile is given in Table 1. The waveguide parameters in Table 1 provide the waveguide fiber properties set forth on Table 2.
A second preferred embodiment is given in Table 3. The waveguide fiber having parameters as set forth in Table 3 also give rise to waveguide fiber properties of Table 2.